Structural insights into the substrate specificity of a glycoside hydrolase family 5 lichenase from Caldicellulosiruptor sp F32

摘要

Glycoside hydrolase (GH) family 5 is one of the largest GH families with various GH activities including lichenase, but the structural basis of the GH5 lichenase activity is still unknown. A novel thermostable lichenase F32EG5 belonging to GH5 was identified from an extremely thermophilic bacterium Caldicellulosiruptor sp. F32. F32EG5 is a bi-functional cellulose and a lichenan-degrading enzyme, and exhibited a high activity on beta-1,3-1,4-glucan but side activity on cellulose. Thin-layer chromatography and NMR analyses indicated that F32EG5 cleaved the beta-1,4 linkage or the beta-1,3 linkage while a 4-O-substitued glucose residue linked to a glucose residue through a beta-1,3 linkage, which is completely different from extensively studied GH16 lichenase that catalyses strict endo-hydrolysis of the beta-1,4-glycosidic linkage adjacent to a 3-O-substitued glucose residue in the mixed-linked beta-glucans. The crystal structure of F32EG5 was determined to 2.8 angstrom resolution, and the crystal structure of the complex of F32EG5 E193Q mutant and cellotetraose was determined to 1.7 angstrom resolution, which revealed that the exit subsites of substrate-binding sites contribute to both thermostability and substrate specificity of F32EG5. The sugar chain showed a sharp bend in the complex structure, suggesting that a substrate cleft fitting to the bent sugar chains in lichenan is a common feature of GH5 lichenases. The mechanism of thermostability and substrate selectivity of F32EG5 was further demonstrated by molecular dynamics simulation and site-directed mutagenesis. These results provide biochemical and structural insights into thermostability and substrate selectivity of GH5 lichenases, which have potential in industrial processes.